Load Redis client from existing environment variable

[lucassz]

Load Redis client from existing environment variable instead of hardcoding an unchangeable hostname for it. Necessary for #913 in an auxiliary way, and seems like a good practice. It's not totally clear to me what the distinction between CELERY_BROKER_URL and CELERY_RESULT_BACKEND (which we set to the same value in practice) is, so do let me know if it should be the other one.

[hdoupe]

@lucassz this looks good to me. Thanks for doing the update.

It's not totally clear to me what the distinction between CELERY_BROKER_URL and CELERY_RESULT_BACKEND (which we set to the same value in practice) is, so do let me know if it should be the other one.

Some where in the celery docs it discusses ways to combine redis and rabbitmq or something like that. However, for us, just using redis gets the job done.

[lucassz]

@hdoupe Thanks for the clarification.

Added a semi-related update to DOCKER.md.

[hdoupe]

@lucassz is #914 ready to merge?

[lucassz]

@hdoupe Yes, looks good to me.

[hdoupe]

Thanks for the contribution @lucassz!